Use of CED's, such as e.g. coiled tubing, sourced from a hydraulically operated reel is known in oil and natural gas exploration and production operations. These tubings, generally refer to metal pipes, e.g. made from steel, with diameter ranging between 1 inch and 4 inches (2.54-10.12 centimeters), or suitably within the range 1.5 to 3.5 inches (3.81-8.89 centimeters). Such tubing may typically have a wall thickness of 5-15% of the tubing diameter, although a different wall thickness range may applied dependent on the use of the tubing. It is also known, that coiled tubing can perform many different oil well operations, and these include use in interventions in oil and gas wells, and use as production tubing in gas wells as well.
Application of such coiled tubing in oil and gas operations involves deploying the tubing as support for drill tools for inserting those tools into boreholes or for retrieving those tools from boreholes. Such tools can be packers, valves, sleeves, sensors, plugs, gauges and so on, which have to be run into and retrieved from the boreholes. These tools may find use for servicing the well.
The operations as stated in the preceding paragraph are done through lubricator string sections and those sections serve as a sluice for undertaking such operations.
How a lubricator string functions for insertion of tools into the well and for retrieval of the same therefrom, are all common knowledge in the art and will not be elaborated any further.
When in use, a coiled tubing injector is normally mounted to an elevated platform above a wellhead or is mounted directly on top of a wellhead. A typically coiled tubing injector is comprised of two continuous belt drive chains, though more than two can be used. The belt drive chains are mounted on sprockets to form elongated loops that counter rotate. A drive system applies torque to the sprockets to cause them to rotate. In most injectors, belt drive chains are arranged in opposing pairs, with the pipe being held between the belt drive chains. Grippers carried by each belt drive chain come together on opposite sides of the tubing and are pressed against the tubing. The grippers, when they are in position to engage the tubing, ride or roll along a skate, which is typically formed of a long, straight and rigid beam. The injector thereby continuously grips a length of the tubing as it is being moved in and out of the well bore. Each skate forces grippers against the tubing with a force or pressure that is referred to as a normal force, as it is being applied normal to the surface of the pipe.
A drive system for a coiled tubing injector includes at least one motor. For larger injectors, intended to carry heavy loads, each belt drive chain will typically be driven by a separate motor. The motors are typically hydraulic, but electric motors can also be used. Each motor is coupled either directly to a drive sprocket on which a belt drive chain is mounted, or through a transmission to one or more drive socket.
During development of injector head it has been found that synchronized chain blocks reduces wear on the continuous elongate device (CED) and prevents fatigue of the continuous elongate device.
With two drive motors the belt drive chains are driven independently of each other on each side of the continuous tubing ie. the chains are driven without synchronization gears. There is a constant torque on the motors.
The chain blocks could in the initial position be synchronized ie the chain blocks from the two independent chains are oppositely positioned in the same horizontal plane. Due to slightly different lengths of the chains, different rotation speed of the drive sprockets etc the chain blocks will after a while be unsynchronized, ie the chain blocks from the two independently chains will not be positioned in the same horizontal plane.
The two oppositely chain blocks will grip the continuous tubing at slightly different time.
This will lead to wear and possible fatigue of the continuous tubing because one the first chain blocks on the first chain will engage with the continuous tubing prior to second chain blocks on the second chain. The first chain blocks will also engage the continuous tubing at another angle than the second chain blocks. This results in that the first chain blocks must travel a longer distance than the second chain. The difference is small typically 0.1 mm per meter continuous tubing, but the difference between the chain blocks could accumulate to an alterations of position or the loss of friction between the continuous tubing and the chain blocks.
To compensate for this uneven rotation of the chains, there have been developed injector head where the chain blocks are rotated synchronously during all movement of the continuous tubing in or out of the well. This has been obtained by using toothed wheel that are mounted on each of the chain drive shafts and interconnected so that both chains are rotated at the same speed and have same position. The toothed wheels are synchronized mechanically in order to obtain the chain block in parallel, opposite positions. This synchronizing of the chain could cause wear and fatigue of the continuous tubing and loss of the lifting force. The chains will rotate around the toothed wheel with the same speed in order to maintain the chains in a synchronized position. This leads to large internal forces within the system due to loss of friction between the chain blocks and the continuous tubing. The chain blocks could slip when they are in contact with the continuous tubing and this could cause damage to the continuous tubing. The synchronized toothed wheel and the chain blocks will in this solution have different speed and are working against each other.
In a system without synchronized toothed wheels the chains will rotate with different speed and the chain blocks will not be synchronized after a while. This will lead to wear and possible fatigue of the continuous tubing because the first chain blocks on the first chain will engage with the continuous tubing prior to second chain blocks on the second chain. The first chain blocks will also engage the continuous tubing at another angle than the second chain blocks. This results in that the first chain blocks must travel a longer distance than the second chain. The difference is small typically 0.1 mm per meter continuous tubing, but the difference between the chain blocks could accumulate to an alterations of position or the loss of friction between the continuous tubing and the chain blocks.